Compensating for distortion in transmission



July 12, '1932.

A. R.A. RENDALL ET AL COMPENSATING FOR'DISTORTION IN TRANSMISSION 3 Filed April 24, i930 -mm suvmsssoz ECHO SUPPRESSGI AIMPUFIER any SYSTEM RELAY svsrem L. DELAT F AMPLIFUERQ M i 5 '50 cannon m a nsrecfoa AMPLIFIER M AMPLIFIER I 2 smock 3:15;. h 4 EQUALIZII}. h lo I i Y E 9" :rzygzmfl Amman AND Ami.

' iquAuzen.

nus: omnusnon A/RA .IPENDALL Y [ZKSAN EMAN 19 provided.

Patented July 12, 1932 I UNITED STATES PATE FF1E,-.

ARTHUR REGINALD ALBERT RENDALL AND EDWARD KENNETH SANDEMAN, E ALDWYCH, LONDON, ENGLAND, ASSIGNORS TO WESTERN ELECTRIC COMPANY, IN- CORPORATER OF NEW YORK, N. Y., A CORPORATION OF NEW YORK i COMPENSATING FOR DISTORTION IN TRANSMISSION Application filed Apri1'24, 1930, Serial No. 446,887, and in Great Britain July 10, 1929.

It is well known that in an ordinary telephone transmission system, frequencies are propagated a-t ditlerentrates so that means are usually provided'ior' compensating for the unequal transmission times in order that faithful reproduction of the messages may be made at the receiving end. Q Thisinvention relates to a telephone transmission system in which such means or phase compensators are In the employment of hase compensating networks it is desirable or reasons of econonly to use the minimum number possible for a given degree of compensation and for quality reasons to insure that as far as possible the proper termination of the networks is provided. Alsothe. networks will not give the value of delay for which they are designed unless perie'ctly'terminated.

According to one feature of the present invention a circuit arrangement comprises two sections each of which. consists of twotransmission paths, one for one direction and the other for the opposite direction, a phase. compensating network being situated in an intermediate path oining said sections and adapted for transmission'in both directions. According to another feature of theinvention in atransmission line employing a repeater of the two wire type, there is provided at a repeater point an additional repeater or an impedance matching network, between which'a'nd the normal repeater is situated a phase compensating network and/orattenu- 'ationequalizer. Other features of the inven- "tion' will be disclosed in the following .description in which Figure 1 illustrates a four wire system modified in accordance with the present invention and Figure 2 illustrates a two wire circuit embodying the principles of the invention. 7 u v V v In the first figure,1, 2, 3, 1 arejfour onecway amplifiers. 5 .and '6 are two hybrid'coils or three winding transformers and may be replaced by any suitable known bridgearrangemerit. 7 is a detector amplifier'operatingrelay'systems 9 and 10, which at appropriate times render inoperativethe east incoming "(ED circuit and the westoutgoing (WO') T circuit; Similarly 8'is a-d'etector amplliiier operating relay systems 11 and 12 which appropriate times render inoperative the west incoming (WI) circuit and the cast outgoingT-(EO)- circuit. I 7, 8, 9, 10,11 and 12 may be replaced by anysuitable known echo suppressors. 13 is the phase compensating networkwhich, as is now evident, is trays t'o permit 7 and 9 to operate before unbalance currents-starting from 1 arriveat 9, a network 1 1 giving a small constant delay may be inserted between 5 and 9 simil arly a delay network 15 may be insertedbetween 6 and 11. In practice it is probable that'these networks will not be necessary, since the major partof any-unbalance currents traversing the hybrid coils will be delayed through having traversed a part or the whole of network 18. The postions in which these networks 14: and 15 are'shown are their preferred positions, notwithstanding the fact that this position ing requires twice asmany sections of delay networksas would be" required were these net: works positioned one at each end of network 13; for thezlatter positioning would require that, for-these networks 1 1'and'15 to fulfill their function, they give" a better balance.

circuit in which the network 13 is located is situated at any point in the middle ofthe V fourwire circuit or at its endsQ An economic arrangmentrls prov ded by locating the net work 13 and its associated apparatus at the point in tlie four' wire circuit where the echo. suppressors normally associated with the four wire circuit are situated, Insuch case these echo suppressors are reprcsented'by 'T, 8,

9,710, 111 and12. Echo suppressors can evi dently be'disposed-in different parts of the circuit to perform the same, function without departing from the spirit of the invention;

7 The methodof operation of the system'is symmetrical and consideration of 'ajconven sation from west to east'will be sufiicient to explain theworking of the whole system. Speech arriving from the circuit WI passes through the amplifier 1 and operating relay systems 9 and 10through the detector amplifier"? paralyzes circuit WC and E1. The speech then proceeds via the hybrid coil 5. The compensating network 13, hybrid coil Grandamplifier 3 to the .circuitEO. The relay system ;9 isintendedito prevent unbals ance speech currents originating from WI passing through the hybrid coil 5 and returnturning via W0 and givingrise to talker echoes. Practically the balance of the hybrid coil to instantaneous transients will bevery highand any currents arriving at 9 from WI will be delayed bylhaving traversed the whole or, part of network 13 twice as a result of reflection from irregularities. For this reason therelay system9 will inpractice have ampletime to operate before any echo currents reach 2. The relay System10 will also haveample time to operatesince theecho path between amplifier 1 and this'point includes the delay network 13 and thereturn circuit from the E0 and EI. It is evident that where other types of distortion are to be .corrected, such as unequal attenuation of different frequencies, the same arrangement can be used with advantage, attenuating correcting networks being inserted in place of or in association with the phase compensatingnetwork 13.

[In Figure 2 is illustratedatwo wire ci r cuit whichhas been modified in accordance with theprincipleof thepresent invention.

Compensating networks of the type which are normallylemployed must be terminated in their characteristic impedance in order thatthe optimumresults may be obtained.

The type of cable circuits normally employed have a characteristic impedance which Varies with frequency, whereas the type o'tphase compensating networks have an impedance which is substantially independent ofjfrequency. For this reasonitiis not convenient to connect phase compensating. networks directly in series with the cable circuit. 7

'In Figure'2 this is avoided by the addition ofanextra repeater, or alternatively, of any other suitable typeof impedance matching network, in circuit'at'the point where a two wire repeater .is normally inserted in the line. The phase compensatingwnetworks are ,thenfinserted between the normal repeater and the secondrepeater or other impedance matching. network. An alternative isthe provisionof two ofthe latter impedance matching'networks in the.circuit,'a't any suitable networksare placed. H V

22 is the ordinary two wire repeater in serted-betweenlines 17 wand 18. .It comprises the amp ifie s B1 n R2. A p as mp sating network 24 has been added in the circuit for the purpose of compensating the different times of transmission of the telephone frequencies over the line 17, 18. In order that a correct m atohing of impedance of the compensatingnetwork 24Lmay be made eon both sides a. second two wire repeater 23 has been inserted as shown comprising amplifiers R .andL-R .If desiredtherepeater.23.maybe substituted by an -impedance matching network .which would. simulate very closely the impedance of the repeater. Thus in the arrangement shown, the-balancing network 19 may be designed to give proper termination to the compensating network 24 and in similar manner the value of the 'balancing network' 20 would be .detemined, and the. impedance pre-. sentedby the repeaters towards :the phase compensating network should ,then be substantially a 'pure'resistanceindependent of;

frequency. On rep-entered circuits where echo. suppressors are employedit. may. beadvantageous to locate the phase compensating networks at the same point as thexechogsupr pressors so that undesirable echoesmaylbe.

eliminated. It is evident that similar arrangements may bezemployed wherecons'tant resistance attenuationequalizers are, required to be used on two wire circuits, the equalizers may then be placedin thesame positionas the phase compensatingnetworks.

In the two wire case (i. e. Fig. 2) if, the balance of the lines 1? and l8ragainst their .respective networks is veryrpoorthe impedances presented-to the network. 24may depart from a pure'resistance. In. such case intolerable departures from 1 the desired delay characteristicmay be obviated byrthe. use of. resistance pads 25 and26insertedlbetween24 and h hybrid coils'oneach .sideofit. V

Whatis. claimedis 7 v1. A telephone transmission. system comprising two. sections with adjacent terminals, each section consisting of Etwo transmission paths,,on e for one directionand the other for the opposite direction :of transmission, and, joining said sections,.an intermediate path [for transmission in both 'directions, -a phase compensating networklbeingsituated in said ,intermediate path.

2. .A; four wire telephone transmissionci-P .cuit comprising,in combination, twosections with adjacent terminals, each section consistingot two transmission .pa'ths one for one directionrand the other for the-opposite .di

rection'of transmission, and, joiningsaid sections, an intermediate path fortransmisslon .in both dlrections, a dlstortion correctlng network being. situated .in said intermediate 7 7 path. point, between which the phase compensating V 3. A telephone -.tr ansmission v system compaths, one .for -,one direction and. the {other for the :opposite -direction of transmission,

and, joining said sections, an intermediate path for transmission in both directions, an attenuation equalizer being inserted in said intermediate path.

5 4. The combination with a telephone transmissionline of the two way, two wire type, of a distortion correcting network and two two-way repeaters at the same point in said line, said repeaters connecting said distortion 3 correcting network therebetween in said line.

5. The combination with a telephone transmission circuit of the two way, two wire type having a characteristic impedance which varies with frequency, of a phase compensating network having an impedance which is substantially independent of frequency, and two two-way repeaters at the same point in said line, said repeaters connecting said phase compensating network therebetween in said 20 line.

6. A four wire circuit according to claim 2, comprising a group of echo suppressors near the middle of the four wire circuit, said distortion correcting network being located at 35 said group of echo suppressors, with certain of the suppressors at one side of the network and certain of the suppressors at the other side of the network. I

In witness whereof, we hereunto subscribe our names this second day of April, 1930.

ARTHUR REGINALD ALBERT RENDALL. EDWARD KENNETH SANDEMAN. 

